Add check for RHEL7/CentOS7 experimental userns disabled

Add a check in `check-config.sh` to see if we are running on a RHEL7 or
CentOS7 system, which may report that CONFIG_USERNS is OK/enabled, but
user namespaces still won't work because of the experimental feature
flag added by Redhat.

This will add a warning if it is actually disabled and notes what has to
be added to the grub/boot command line to enable it.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)